1. Field of the Invention
The present invention relates to a novel flower-bud formation suppressor gene which is useful to confer an early flowering property on a perennial plant. The present invention also relates to a recombinant vector which comprises a DNA encoding the above gene or an antisense DNA of the gene, a transformant which comprises the recombinant vector, and seeds obtained from the transformant. The present invention further relates to a method for conferring an early flowering property on a plant, and a method for producing an early flowering plant.
2. Description of the Related Art
Unlike for herbaceous plants, it takes a long time for woody plants to progress from flowering to seed-setting, that is, woody plants have long juvenile periods. Hence, improvement through breeding of a perennial fruit tree crop, such as Malus (apple) belonging to the family Rosaceae, requires a long period of time. For example, an apple needs a 7 to 8 year-period from sowing to first seed-setting. In breeding of fruit trees, when an agriculturally useful gene, such as a disease-resistant gene of a wild species, is introduced into a cultivar, nearly 10 times of crossing are stochastically essential to obtain a good quality line. Thus, juvenility, the unique feature of woody plants, is a significant factor that impedes the efficiency of cross breeding.
Conventional means to shorten the breeding period of a fruit tree, such as early flowering and seed-setting, use of dwarfing stocks, and training of fruit-bearing mother branch, have been attempted. However, no method can be said to have exerted any landmark effect (Pinero, M. et al., Plant Physiol. 1998, 117: 1–8; Levy, Y. Y. et al., Plant Cell 1998, 10: 1973–1989).
On the other hand, in recent years, several genes regulating flower-bud formation has been isolated from Arabidopsis thaliana which is a model plant for Dicotyledon (Ohshima, S. et al., Mol. Gen. Genet. 1997, 254(2): 186–194; Bradley, D. et al., Scicence 1997, 275(5296): 80–83). The molecular mechanism of flower-bud formation is now being elucidated. For example, TERMINAL FLOWER 1 gene (TFL1) has been known to be a gene which is capable of suppressing flower-bud formation (late-flowering genes) (International Patent Publication No. WO 97/10339). In Arabidopsis thaliana, it has been experimentally shown that overexpression of the gene causes late flowering, and suppressed expression thereof causes early flowering (Ratcliffe, O. J. et al., Development 1999, 126: 1109–1120; Liljegren, S. J. et al., Plant Cell 1999, 11: 1007–1018). However, genes having a similar sequence with TFL1 but having a reverse effect are also known. So it has not been easy to clarify the genes and mechanisms involved in the flowering (Weigel, D. et al., Nature 1995, 377: 495–500; Mandel, M. A. et al., Nature 1995, 377: 522–524). Moreover, neither a gene nor a protein involved in the flower-bud formation in Rosaceous plants including the genus Malus has been shown to date.
Further in the field of agriculture, attempts, such as to confer an early flowering property on perennial crops by regulating genes that suppress flower-bud formation, have not been reported so far.